Nondestructive assessment of static and dynamic geomechanical properties of sandstone samples for geological CO_2 storage monitoring

摘要

We present an experimental nondestructive approach to evaluate the geomechanical response of a rock sample through static and dynamic methods, simultaneously. The technique uses an increasingly differential stress path to determine the static and dynamic elastic moduli and effective stress coefficients of the sample, but without causing permanent damage. In this study, we use synthetic sandstone with nominal porosity of 0.38, particularly selected to simulate an induced overpressure scenario in a shallow offshore reservoir, associated with CO_2 storage practices. The experiment is replicated for dry and brine saturated conditions, showing the weakening effect of the water in the elastic moduli. The dry Young's modulus obtained from unconfined compressive strength (UCS) tests is similar to those derived from our tests, for both the dynamic and static cases; the static Poisson's ratios are about 30% higher than expected according to the UCS, but are in closer agreement for the dynamic case. Furthermore, ultrasonic wave velocities (V_p and V_s) vary slightly (1%) for deviatoric stress up to 9 MPa, but there is a negligible effect on the bulk modulus. Using dynamic parameters to compute the static effective stress coefficient (n) returns values close to Biot's coefficient (0.8 - 0.85). However, n increases to 0.95 when the calculation is based on axial strain (i.e., static parameter). Overall, the technique provides a valuable geomechanical baseline, useful for comparing static and dynamic parameters of the rock and calibrating key inputs for geomechanical models.